1. Technical Field
The present invention relates to a technique which is useful for the detection of a pulsed signal. In particular, the present invention relates to a technique for measuring a distance by utilizing a reflection of a pulsed light, and relates to a technique for detecting a faint reflected pulsed light at a high sensitivity.
2. Related Art
A technique is known for measuring a distance to an object by irradiating a pulsed light to the object and detecting a reflected light. This technique is called a “light wave distance measuring technique” and is a technique for determining the distance to an object on the basis of a time difference between an irradiation timing of the pulsed light to the object and a detection timing of the reflected light. The technique is described, for example, in patent document 1 (Japanese Patent National Publication of Translation No. 2000-506971) and patent document 2 (Japanese Patent No. 3089332).
The light wave distance measuring technique mentioned above includes a method using a reflected light from a reflector for exclusive use called a “cube corner reflector”, and a method using a reflected light from an optionally selected object without using the cube corner reflector. The method using no cube corner reflector is easier to use because it uses a simple structure using no cube corner reflector; however, since the reflected light is weak, a device is necessary for detecting the pulsed light.
In the distance measuring technique, it is desired to measure at a higher accuracy and measure in a larger range (a larger measurement range). In the case of improving accuracy of measurement data, it is necessary to make a sampling interval of an output from a light receiving element shorter. However, making the sampling interval short requires an increase in sampling data. This matter appears to be significant in the case in which the measurement range is enlarged.
Since it is preferable to measure the distance in the shortest time possible, it is not preferable that the sampling data be increased. Furthermore, there is a problem in that the increase of the sampling data increases a load on a memory storing the data and an arithmetic circuit, thereby incurring a high cost and an increase in electric power consumption. The increase of the electric power consumption is significantly disadvantageous in that the light wave distance measuring apparatus is frequently used outdoors and requires a construction which can be driven by a battery. Furthermore, there is a problem in that a restriction on a circuit design is increased by increasing sampling frequency, and a high cost is incurred.
Furthermore, in the measurement using no cube corner reflector mentioned above, since an intensity of the reflected light is weak, it is necessary to devise it so as to improve detection accuracy of the reflected pulsed light. As this technique, there has been known a process for temporarily storing the sampling data of the reflected pulsed light at plural times, and accumulating the stored sampling data for a plurality of pulses (for example, for ten pulses) in a state of positioning the data on a time axis. In this process, there is executed an arithmetic process of accumulating the sampling data at the same sampling position in a plurality of pulses and improving an S/N ratio. In other words, there is executed an arithmetic process of overlapping a plurality of pulses.
However, this method requires processing time for processing the pulses at plural times, and has a problem in that measuring time is increased. Since the data amount to be processed is increased in the case of enlarging the measurement range, this tendency becomes more serious.
As mentioned above, when seeking out the measuring accuracy in spite of the restriction in the processed sampling data, it is unavoidable to make a sacrifice of the measurement range. On the other hand, in the case of seeking out the magnitude of the measurement range, it is unavoidable to widen the sampling interval, so that the measuring accuracy is sacrificed. Furthermore, in the system using no cube corner reflector, a predetermined measuring accuracy can be obtained only by further making a sacrifice of the measurement range in addition thereto, so that there is additionally a problem that the measuring time becomes long. As mentioned above, a point to be improved exists in the light wave distance measuring technique.
The patent document 1 describes a technique for executing a rough measurement utilizing one to three pulses and a precise measurement utilizing a plurality of pulses. The technique corresponds to one of the approaches for improving the problems mentioned above; however, in the case in which the reflected light is weak without using the cube corner reflector, there is a disadvantage that a sufficient detecting output cannot be obtained in the rough measurement, and the signal fails to be picked up.